Photopolymerizable compositions are widely used for an increasing number of usages. For example this type of composition is now commercially used in printing, copying resist formation, etc. Such compositions generally contain an ethylenically unsaturated compound or other type polymerizable compound, a photoinitiator or photoinitiator system and preferably a solvent-soluble or aqueous or alkaline soluble organic polymeric binder compound. Many of the known useful photopolymerizable compositions, however, are limited in applicability because the initiators are not as active as desired.
Chambers U.S. Pat. No. 3,479,185 discloses photopolymerizable compositions containing an ethylenically unsaturated monomer, a free radical producing agent such as an amine substituted leuco triphenylamine dye and a hexaarylbiimidazole. These compositions are photoinitiatable in the ultraviolet region of the spectrum. Chambers, however, found that by adding energy-transfer dyes of the xanthene and acridine classes the sensitivity of the photopolymerizable compositions was extended into the visible spectral region with an increase in speed of polymerization.
Chang U.S. Pat. No. 3,549,367 discloses photopolymerizable compositions containing hexaarylbiimidazoles and p-aminophenyl ketones, e.g., Michler's ketone, which increase the speed of polymerization of the compositions.
It is generally assumed that photosensitization by Michler's ketone, e.g., Chang U.S. Pat. No. 3,549,367, occurs via the triplet state; see also D. I. Schuster, M. D. Goldstein and P. Bane, J. Amer. Chem. Soc., 99, 1977, pages 187-193. When a molecule absorbs light the exciting energy may be partitioned through various pathways among triplet and singlet states. This is shown diagrammatically by a Jablonski diagram as given by R. O. Kan, "Organic Photochemistry," McGraw-Hill Book Co., NY 1966 page 12, and explained by J. B. Birks, "Photophysics of Aromatic Molecules," Wiley-Interscience, London, 1970, pages 30-32, and N. J. Turro, "Modern Molecular Photochemisry, Benjamin/Cummings Publishing Co., Menlo Park, CA, 1978, page 5 (FIG. 1.2). From the diagrams it can be seen that any pathway that serves to prevent the triplet state from becoming populated, either by shortening the lifetime of the triplet state, e.g., by phosphorescence, or preventing the formation of the triplet state, e.g., by fluorescence, will decrease the usefulness of the compound as a photosensitizer. The quantum yield of phosphorescence for the compounds relative to Michler's ketone is determined by the formula: .phi.p(X)/.phi.p(Mk) (Michler's ketone value is therefore 1.0). The compounds found to have relative quantum yields of phosphorescence greater than 1.0 exhibit a shorter triplet state lifetime than the triplet state lifetime of Michler's ketone (I. B. Berlman, "Handbook of Fluorescence Spectra of Aromatic Molecules," second edition, Academic Press New York, 1971 page 41, formula 16, and N. J. Turro, ibid, page 90, formula 5.23 and page 109, formula 5.42). The compounds with higher relative quantum yield of phosphorescence would not be expected to photosensitize addition polymerization as well as Michler's ketone because the particular triplet would not live as long as the triplet of Michler's ketone and therefore not have the same opportunity to transfer its triplet energy before it lost it through phosphorescence. The presence of additional rings or bridges on Michler's ketone causes the molecules to have moderate to strong fluorescence, while under the conditions employed, Michler's ketone was found not to fluoresce. It would appear that the presence of additional rings or bridges on Michler's ketone is unexpectedly causing profound alteration of the photophysics of these molecules similar to the known alteration of energy levels in Michler's ketone itself by changes in solvent (D. I. Schuster, et al., idem.). Since fluorescence is a pathway whereby the excited singlet returns to the ground state without passing through the triplet state, compounds which show increased fluorescence at 77.degree. K. and room temperature would not be expected to sensitize as effectively as Michler's ketone. Fishman U.S. Pat. No. 3,552,973, column 3, lines 60 to 62, states that his disclosed ketone sensitizers "are naturally phosphorescent in rigid media at low temperatures, such as in ether-isopentane alcohol glass at 77.degree. K." In addition, none of the compounds that Fishman discloses are naturally fluorescent under these conditions. It would therefore be expected that the triplet state of Fishman's disclosed compounds at low temperature or in rigid media would be more highly populated since energy wasting fluorescence is not present.
While the aforementioned compositions have provided improved photopolymerizable compositions, there is a need to provide photopolymerizable compositions having improved photospeed.